Apparatus for controlling the fabrication of electroexplosive devices

ABSTRACT

A method and apparatus for controlling the fabrication of electroexplosive devices. An electroexplosive device is loaded with an explosive by pressure loading or ignition drop or other techniques, the explosive surrounding an electroexplosive bridgewire located within the device. A self-balancing bridge is connected to the bridgewire to monitor the bridgewire thermal conductance during loading. In one embodiment of the invention, an indicator is connected to the self-balancing bridge to display the variation in thermal conductance providing an indication of the thermal contact between the explosive and the bridgewire. In an alternative embodiment, the output of the self-balancing bridge is coupled to a comparator which compares the bridgewire thermal conductance to a desired value. The output of the comparator is an error signal which is amplified in an amplifier connected thereto, the amplified error signal being coupled to a control device to control the loading of the electroexplosive device and provide consistent and improved thermal contact between the explosive and the bridgewire.

United States Patent Kabik et al.

July 3, 1973 APPARATUS FOR CONTROLLING THE FABRICATION OF ELECTROEXPLOSIVE DEVICES [75] Inventors: Irving Kabik, Silver Spring, Md.;

Louis A. Rosenthal, Middlesex County, NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, D.C.

[22] Filed: Aug. 13, 1970 [21] Appl. No.: 63,378

[52] US. Cl. 86/20 R, 29/593 [51] Int. Cl C06d l/08 [58] Field of Search 324/65; 29/593, 610, 29/623; 425/170, 149; 86/1, 20 R [56] References Cited UNITED STATES PATENTS 3,028,528 4/1962 Cheselin, Jr 324/65 B 3,175,206 3/1965 Lindberg, Jr. 324/65 3,186,228 6/1965 Lever et al 324/65 2,149,827 3/1939 Andre 29/623 3,044,139 7/1962 Morton et al 425/170 X 3,067,465 12/1962 Giardini et a1..... 425/170 X 3,426,643 2/1969 Dillehay 86/1 R 3,559,247 2/1971 Lars son 425/149 3,389,459 6/1968 Rousel................................ 29/6l0 FOREIGN PATENTS OR APPLICATIONS 581,474 8/1959 Canada 29/593 Primary Examiner-Charles W. Lanham Assistant Examiner-Robert M. Rogers Att0rneyR. S. Sciascia and J. A. Cooke ABSTRACT A method and apparatus for controlling the fabrication of electroexplosive devices. An electroexplosive device is loaded with an explosive by pressure loading or ignition drop or other techniques, the explosive surrounding an electroexplosive bridge-wire located within the device. A self-balancing bridge is connected to the bridgewire to monitor the bridgewire thermal conductance during loading. In one embodiment of the invention, an indicator is connected to the self-balancing bridge to display the variation in thermal conductance providing an indication of the thermal contact between the explosive and the bridgewire. In an alternative embodiment, the output of the self-balancing bridge is coupled to a comparator which compares the bridgewire thermal conductance to a desired value. The output of the comparator is an error signal which is amplitied in an amplifier connected thereto, the amplified error signal being coupled to a control device to control the loading of the electroexplosive device and provide consistent and improved thermal contact between the explosive and the bridgewire.

6 Claims, 5 Drawing Figures SENSOR INDICATOR PAIENTEDJUU I915 3.742.811

INVENTORS Irving Kabik Louis A. Rosemhal ATTORNEY APPARATUS FOR CONTROLLING THE FABRICATION OF ELECTROEXPLOSIVE DEVICES BACKGROUND OF THE INVENTION This invention relates generally to electroexplosive devices and, more particularly, to a method and apparatus for controlling the fabrication of electroexplosive devices.

One of the first and most critical steps in the manufacture of electroexplosive devices, such as detonators or the like, is the application of a primary explosive to the bridgewire of the electroexplosive device. It is essential for proper operation ofthe electroexplosive device that good thermal contact exist between the primary explosive and the bridgewire since the firing of the electroexplosive device is actuated by heat transfer from the bridgewire to the primary explosive which is initiated by allowing an electric current to flow through the bridgewire, the latter being heated by the electric current.

Prior art methods for fabricating electroexplosive devices have assumed that a good thermal contact, required for proper operation of the electroexplosive device, can be obtained by pressure loading the primary explosive around the bridgewire. In such methods of fabrication, a hydraulic press, or the like, forceably inserts the primary explosive into the electroexplosive device until a predetermined pressure is reached, it being assumed that good thermal contact will exist between the primary explosive and the bridgewire when the predetermined pressure is obtained. Unfortunately this prior art pressure monitoring method of fabrication is unable to insure that good thermal contact exists between the primary explosive and the bridgewire. This is due, in part, to chemical binders in the primary explosive 'which frequently coat the bridgewire at the predetermined pressure or to oxidation of the bridgewire itself or to other factors which prevent the loading pressure from being an adequate parameter to monitor in attempting to achieve good thermal contact between the explosive and the bridgewire.

Other prior art methods for fabricating electroexplosive devices utilize the ignition drop technique wherein the primary explosive is in the form of a liquid slurry, such as, for example, a lacquer, the slurry being deposited on the bridgewire and allowed to cure in the electroexplosive device.

The heretofore employed methods for fabricating electroexplosive devices, whether utilizing pressure loading or ignition drop or other techniques, have been unable to provide electroexplosive devices exhibiting a uniform thermal contact between the explosive and the bridgewire from device to device and, as a result thereof, the explosive properties of the electroexplosive devices have varied.

One solution for obtaining consistent electroexplosive devices is shown in U. S. Pat. No. 3,374,429 issued to L. A. Rosenthal wherein an apparatus and method for determining the thermal parameters of bridgewires used in electroexplosive devices is disclosed. The method and apparatus disclosed therein is adequate for measuring the thermal contact between the explosive and bridgewire for fully fabricated electroexplosive devices. However this heretofore-identified patent does not disclose a method for monitoring the thermal contact during actual fabrication at which time control of the thermal characteristics is most easily achieved.

BRIEF SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to obtain an electroexplosive device exhibiting good thermal contact between the primary explosive and the bridgewire of the device.

Another object of the present invention is to provide electroexplosive devices exhibiting consistent thermal contact.

A further object of the present invention is to provide a method for fabricating electroexplosive devices.

Another object of the instant invention is to provide an apparatus for measuring the thermal parameters of electroexplosive devices.

Briefly, these and other objects of the present invention are attained by providingan apparatus and method for monitoring the thermal conductance of a bridgewire of an electroexplosive device during the introduction of a primary explosive around the bridgewire. The change in bridgewire environment, caused by introduction of the primary explosive, results in a change in the thermal parameter of the bridgewire. In one embodiment of the invention, the changing thermal parameter is monitored as an indication of thermal contact between the explosive and the bridge-wire. In analternative embodiment, the thermal parameter is monitored to provide a control signal for regulating the fabrication process.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the invention and many of the attendant advantages thereof will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a block diagrammatic view of a system for monitoring the thermal characteristics of electroexplosive devices according to one embodiment of the present invention;

FIG. 2 is a schematic diagrammatic view of the sen sor utilized in FIG. 1;

FIG. 3 is a graphical view of the signal characteristics of the sensor of FIG. 2;

FIG. 4 is a circuit diagrammatic view of the sensor of FIG. 2; and,

FIG. 5 is a block diagrammatic view of a system for controlling the thermal characteristics of electroexplosive devices according to an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and more particularly to FIG. 1 thereof, an electroexplosive device, such as a detonator, or the like, is indicated at 10 as including a housing or the like 12. A bridgewire 14 is disposed within housing 12 and is adapted to actuate the electroexplosive device by transferring heat to aprimary explosive 16 surrounding the bridgewire in response to current flow through the bridgewire. As hereinbefore explained, primary explosive 16 may be in the form of a liquid slurry introduced into housing 12 through an applicator or guide or the like 18 and allowed to cure therein. It is readily apparent, of course, that primary explosive 16 may also take the form of a powder or the like introduced into housing 12 and pressed under pressure therein by a conventional plunger or the like, driven by a motor (not shown).

As hereinafter more fully explained, an indication of thermal contact may be obtained by monitoring the heat loss factor or thermal conductance, -y, in a sensor 20 coupled to the bridgewire. The output from sensor 20 is connected to a conventional indicator 22 adapted to provide a reading related to thermal conductance and, therefore, an indication of thermal contact between bridgewire l4 and explosive 16.

The response of a bridgewire to various electrical signals applied thereto depends on the thermal characteristics of the bridgewire, that is, the heat capacity, C,,, the thermal conductance or heat loss factor, and the thermal time constant, 7, given by the relation r= C,,/'y. The thermal conductance, 'y, is a convenient parameter to monitor and provides an indication of the thermal contact and, therefore, an indication of the responsiveness of the bridgewire to signals applied thereto.

A simple thermal model for the response of the bridgewire based on a lumped single time constant system is provided by the following basic differential equation:

0, (dO/dt) y a P(t) wherein C, is the heat capacity of the bridgewire, -y is the thermal conductance, P(t) is the power time function, and 0 is the temperature rise of the bridgewire.

For the steady state condition, equation (1) reduces to since (dB/dz) 0.

Consider a bridgewire of resistance r,,, as the temperature of the bridgewire varies, for example due to current flow therethrough or, as hereinafter explained, due to changes in the physical environment of the bridgewire, the resistance r,, will change according to the relation:

wherein AR is the change in resistance of the bridgewire due to a temperature change 0.

the change in bridgewire resistance, AR, constant. With AR constant, equation 4(b) may be rewritten as:

k 'y P(t) wherein k-= AR/aR, constant. As indicated by equation (5), a direct and linear relationship exists between the thermal conductance, 'y, and the power input to the bridgewire if the resistance change AR is kept constant for the thermal differential equation heretofore described.

Reference to FIG. 2 of the drawing shows a schematic diagram of a self-balancing bridge utilized to keep the resistance change, AR, constant as the temperature of the bridgewire varies an amount 0. The selfbalancing bridge includes a conventional selective amplifier 24 adapted to provide an amplifier gain A at a tuned frequency f,,, coupled to a thermally responsive bridge 26. The thermally responsive element in bridge 26 is provided by the electroexplosive bridgewire 14 which is connected as one arm of the bridge, the resistance of which varies with temperature according to equation (3). Resistors 28, 30, and 32 of variable resistance R fixed resistance R and fixed resistance R respectively, form the other three arms of bridge 26. Resistors 28, 30, and 32 are advantageously chosen with very small temperature coefficients of resistivity and, therefore, the resistance values thereof are relatively insensitive to changes in temperature.

The output of amplifier 24 is connected to bridge 26 and is adapted to provide an input signal, e to the bridge which is applied between the juncture of resistors 30 and 32 and the grounded juncture of resistors 14 and 28. As in conventional bridge networks, when the bridge is balanced no signal will appear between the juncture of resistors 28 and 30 and the juncture of resistors 14 and 32. At unbalance, an error signal, s will appear between the aforementioned two junctures, the error signal being applied to the input of amplifier 24 as a positive feedback signal thereto.

Referring now to FIG. 3, the bridge input, e versus the error signal, e,,, is shown for the self-balancing bridge of FIG. 2. The bridge is balanced when the relationship A (It/R.) mm.)

is satisfied, wherein R,, R and R are the temperature insensitive resistance values of resistors 28, 30, and 32, respectively, and r,, is the temperature dependent resistance of bridgewire 14.

Assuming, initially, that the value of the bridgewire resistance, r,,, is equal to the cold wire resistance, R,,,- and that the. value of resistor 28 is adjusted so that the relationship is satisfied, then the bridge will be unbalanced. If an input signal, e,,,, is applied to the bridge, current will flow therethrough which will raise the temperature of resistors 28, 30, and 32 and bridgewire 14, however, only the resistance value of the bridgewire is affected by the temperature change since the other resistors are temperature insensitive. The bridgewire resistance, r,,, will increase from its cold wire value, R to a new value, R as given by equation (3);. This increase in bridgewire resistance brings the bridge closer to balance since the ratio r,,/R will increase from R,,/R to [R and the inequality of equation (6b) will decrease.

The bridge cannot become balanced immediately, however, and since the bridge was initially unbalanced, an error Signal, e,,, will appear at the juncture of resistors 28, 30 and 14, 32, respectively. The error signal is coupled to the input of amplifier 24, as positive feedback thereto, and is amplified, thereby increasing the amplifier output and, consequently, increasing the signal, e applied to bridge 26. The increased signal, e supplied to bridge 26 raises the temperature of the bridgewire resulting in an increase in the bridgewire resistance, r bringing the bridge closer to balance. The cycle is repeated wherein the new error signal augments the applied bridge voltage causing a further increase in bridgewire temperature and a corresponding increase in bridgewire resistance bringing the bridge closer to balance. Complete balance can never be obtained, however, for an amplifier with a finite gain A since, at balance, feedback would cease and the amplifier would stop oscillating.

Referring to FIG. 3, theoretical complete balance is indicated as point B therein corresponding to zero error signal. As a practical matter, the bridge approaches complete balance at a point B wherein the error signal, e,,, is approximately zero, and the bridgewire resistance, r,,, has increased in value from its cold wire resistance, R,,, to a new value, R (balance) wherein the relation i/ R (balance)/R,

is approximately satisfied. It is to be noted that the slope of the line to point B is the reciprocal of the amplifier gain A. Thus, the self-balancing bridge of FIG. 2 increases the value of bridgewire resistance, r,,, from its cold wire value, R to a resistance, R9 (balance), required to balance the bridge by electrically heating the wire according to equation (3). More particularly, the resistance change, AR R Ra, preset by adjusting the resistance values R,, R and R is held constant and, therefore, the self-balancing bridge is an apparatus which satisfies equation (5). As hereinafter more fully explained, if the physical environment of bridgewire 14 changes causing the bridgewire to cool in temperature and, therefore, causing a corresponding drop in bridgewire resistance from R (balance), the bridge will become unbalanced and an error signal, e will appear. The error signal, e,,, will be fed back to amplifier 24, increasing the applied signal to the bridge, e to raise the temperature of the bridgewire and, therefore, raise the resistance to R (balance) resulting in bridge balance.

FIG. 4 shows a more complete schematic diagram of a self-balancing bridge utilized to keep the resistance change, AR, of bridgewire l4 constant. The selfbalancing bridge includes a thermally responsive bridge 34, similar to bridge 26 of FIG. 2. Positive feedback is provided from bridge 34 to a conventional amplifier stage 36 via a lead 38 connected therebetween. Connected to amplifier 36 is a conventional tuned amplifier 40 which is adjusted to provide amplification about a frequency, f,,. The value of f, is chosen to be of the order of 1,000 hertz which is a compromise between low frequencies which would result in thermal follow and higher frequencies which would require the inclusion of reactive balance components in bridge 34. A conventional driver stage 42 is coupled between frequency selective amplifier 40 and a push-pull output stage 44. Power is supplied to the amplifier-bridge network from a conventional power supply 46. The pushpull output stage 44, which 'drives the thermal bridge 34, is advantageous since the power signal supplied to the bridge is clipped due to saturation of the push-pull stage. As a result thereof, the total maximum power that is transferred from the push-pull stage to bridgewire 14 is kept below a predetermined limit and, therefore, the possibility that the supplied power may accidentally actuate the electro-explosive device is alleviated.

As hereinbefore explained, it is desirable to measure the thermal conductance, 'y, of the electroexplosive bridgewire since the thermal conductance provides an indication of the thermal contact between the bridgewire and the primary explosive surrounding the bridgewire. One method for controlling the fabrication of electroexplosive devices utilizing thermal conductance is shown in FIG. 1..As indicated therein, the thermal conductance is monitored during device fabrication by a sensor 20 and indicator 22. Sensor 20 may be similar to the self-balancing bridge of FIGS. 2 and 4, it being understood, however, that other sensors for monitoring thermal conductance may be utilized if so desired. Similarly, other thermal parameters, such as, for example the heat capacity, C,,, or the thermal time constant, 1-, may advantageously be monitored.

Initially, bridgewire 14 is at its cold wire resistance value, R,,, and housing 12 is divorced from any primary explosive, that is, no primary explosive surrounds the bridgewire. The self-balancing bridge supplies power to the bridgewire raising the bridgewire temperature, as hereinbefore explained, until equation 6(c) is satisfied and the bridgewire resistance reaches R9 (balance). If primary explosive 16 is now introduced into housing 12, the physical environment of the bridgewire changes and, more particularly, the bridgewire is cooled due to the thermal conductivity of the primary explosive which is now in contact with the bridgewire. As a result thereof, the resistance of the bridgewire drops from R (balance) and, consequently, the bridge becomes unbalanced. Automatically, the self-balancing bridge increases the power supplied to the bridgewire to increase the temperature of the bridgewire and return the bridgewire resistance to R9 (balance). The power supplied to the bridgewire is measured, for example, by measuring the input to the bridge, e or by connecting a watt-meter to the bridgewire, and, as indicated by equation (5), the power supplied to the bridgewire is directly proportional to the bridgewire thermal conductance. It is readily apparent that by monitoring the power supplied to the bridgewire and, therefore, moni toring the thermal conductance, 'y, and the variation thereof during the important step when the primary explosive is added to the bridgewire, a more precise indication of thermal contact between the bridgewire and the explosive is obtained.

An alternative embodiment of the invention advantageously utilizing the thermal conductance, 'y, as a control parameter is illustrated in FIG. 5. As indicated therein a loader 48, such as a hydraulic press or the like, is adapted to supply an electroexplosive device 10 with a primary explosive (not shown). More particularly, press 48 is adapted to pack a primary explosive into a housing to surround an electroexplosive bridgewire. As hereinbefore explained, as the physical environment of the bridgewire is varied, for example, due to the increased pressure of the primary explosive, the thermal conductance, -yof the bridgewire changes. The variation in bridgewire thermal conductance is sensed in sensor coupled to the electroexplosive device and the thermal conductance is compared with a predetermined value provided by a reference 50 in a comparator 52 connected to the reference 50 and sensor 20. The output of the comparator is an error signal, which is amplified by a conventional amplifier 54 coupled between the comparator and a control network 56, and is utilized to control the pressure loading of the explosive into the electroexplosive device. Control network 56 is a conventional device, which may be an on-off switch, adapted to vary the power applied to hydraulic press 48 according to the error signal input thereto. Thus, the loading pressure is controlled by utilizing the thermal conductance, 'y, as a control parameter to obtain good thermal contact between the explosive and the bridgewire.

It is readily apparent, therefore, that the apparatus and method for controlling the fabrication of electroexplosive devices provides a reliable and consistent electroexplosive device by providing excellent thermal contact therein. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, other techniques for measuring thermal conductance, such as, for example, thermal follow and cooling curve measurements, may be utilized. Furthermore, the thermal conductance, y, may be utilized as an electrothermal parameter and used as an end-point control or as an automatic cut-off or the like in other techniques of loading rather than the pressure and the ignition drop methods hereinbefore described.

Still further, other thermal parameters related to thermal contact, such as the heat capacity, C,,, and the thermal time constant, 1-, may be monitored. It is to be therefore understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is new and desired to be claimed by LettersPatent of the United States is:

1. A system for controlling the fabrication of electroexplosive devices comprising:

explosive means; means for loading said explosive into an electroexplosive device; a bridgewire located within said electroexplosive device; means for sensing a thermal parameter of said bridgewire during loading of said device to provide an indication of the thermal contact between said bridgewire and said explosive. 2. A system for controlling the fabrication of electroexplosive devices according to claim 1 wherein said thermal parameter is the thermal conductance of said bridgewire. 3. A system for controlling the fabrication of electroexplosive devices according to claim 2 wherein said means for sensing the thermal conductance of said bridgewire provides power to the bridgewire which satisfies the relationship wherein P(t) is the power supplied to said bridgewire, 'y is the thermal conductance of said bridgewire, a is the temperature coefficient of resistivity of said bridgewire, AR is the resistance change in said bridgewire from the bridgewire cold resistance, R,,, due to temperature change in said bridgewire.

4. A system for controlling the fabrication of electroexplosive devices according to claim 3 wherein said means for sensing the thermal conductance of said bridgewire includes means for keeping said resistance change of said bridgewire constant during loading of said electroexplosive device.

5. A system for controlling the fabrication of electroexplosive devices according to claim 4 wherein said means for sensing thermal conductance is a selfbalancing bridge.

6. A system for controlling the fabrication of electroexplosive devices according to claim 2 further comprismeans for comparing a signal proportional to said sensed thermal parameter with a signal proportional to a predetermined value to obtain an error signal therebetween, and

means to vary the loading of said electroexplosive device proportional to said error signal to control said thermal contact between said bridgewire and said primary explosive. 

1. A system for controlling the fabrication of electroexplosive devices comprising: explosive means; means for loading said explosive into an electroexplosive device; a bridgewire located within said electroexplosive device; means for sensing a thermal parameter of said bridgewire during loading of said device to provide an indication of the thermal contact between said bridgewire and said explosive.
 2. A system for controlling the fabrication of electroexplosive devices according to claim 1 wherein said thermal parameter is the thermal conductance of said bridgewire.
 3. A system for controlling the fabrication of electroexplosive devices according to claim 2 wherein said means for sensing the thermal conductance of said bridgewire provides power to the bridgewire which satisfies the relationship P(t) ( gamma Delta R)/( Alpha Ro) wherein P(t) is the power supplied to said bridgewire, gamma is the thermal conductance of said bridgewire, Alpha is the temperature coefficient of resistivity of said bridgewire, Delta R is the resistance change in said bridgewire from the bridgewire cold resistance, Ro, due to temperature change in said bridgewire.
 4. A system for controlling the fabrication of electroexplosive devices according to claim 3 wherein said means for sensing the thermal conductance of said bridgewire includes means for keeping said resistance change of said bridgewire constant during loading of said electroexplosive device.
 5. A system for controlling the fabrication of electroexplosive devices according to claim 4 wherein said means for sensing thermal conductance is a self-balancing bridge.
 6. A system for controlling the fabrication of electroexplosive devices according to claim 2 further comprising means for comparing a signal proportional to said sensed thermal parameter with a signal proportional to a predetermined value to obtain an error signal therebetween, and means to vary the loading of said electroexplosive device proportional to said error signal to control said thermal contact between said bridgewire and said primary explosive. 